1. Field of the Invention
The present invention relates to a holder furnace for containing a supply of molten metal and, more particularly, to a bottom heated holder furnace that may be used as part of a molten metal casting system.
2. Description of the Prior Art
Molten metal holding furnaces, or holder furnaces, are used in the art for holding and/or melting molten metal. Holding furnaces are often used to contain a supply of molten metal for injection into a casting machine. For example, U.S. Pat. No. 4,753,283 to Nakano discloses a horizontal injection casting machine in which molten metal is maintained in a holding furnace which periodically provides molten metal to the casting machine. Molten metal from a larger smelting furnace is supplied periodically to the holding furnace to maintain a set amount of molten metal in the holding furnace. The holding furnace is heated by a burner located adjacent a sidewall of the holding furnace.
In addition to the burner arrangement disclosed by the Nakano patent, several other methods are known in the art for heating molten metal contained in a holding furnace. Several common methods include induction heating, radiant heating, and immersion heating. For example, U.S. Pat. No. 4,299,268 to Lavanchy et al. discloses a molten metal casting arrangement in which molten metal is contained in a large capacity pressure ladle (i.e., holding furnace) that is heated by a heating inductor located at the bottom of the pressure ladle. The pressure ladle periodically supplies molten metal to a smaller capacity tilting ladle, which supplies molten metal to a casting apparatus. U.S. Pat. No. 3,991,263 to Folgero et al. discloses a similar molten metal holding system to that disclosed by the Lavanchy et al. patent, but the system disclosed by the Folgero et al. patent is pressurized.
U.S Pat. No. 4,967,827 to Campbell discloses a melting and casting apparatus in which electric radiant heating elements are used to heat molten metal passing from a holding furnace to a casting vessel. U.S. Pat. No. 5,398,750 to Crepeau et al. discloses a molten metal supply vessel in which a plurality of electric immersion heaters is used to heat molten metal in a holding furnace. The immersion heaters extend downward from the holding furnace cover and are partially submerged in the molten metal contained in the holding furnace. U.S. Pat. No. 5,567,378 to Mochizuki et al. discloses a similar immersion heater arrangement to that found in the Crepeau et al. patent.
The above-discussed radiant heating and immersion heating elements for heating molten metal in a holding furnace are located above the surface of the molten metal and are xe2x80x9ctopxe2x80x9d heating arrangements. The xe2x80x9ctopxe2x80x9d heating arrangements known in the art require a significant amount of space above the holding furnace for the individual heating elements. For example, the immersion heaters and electric radiant heaters discussed previously in connection with the Crepeau et al. and Campbell patents require a significant amount of space above the surface of the molten metal in the holding furnace, as well as a support structure above the holding furnace for supporting the heating elements above the surface of the molten metal. External heating arrangements, such as the burner arrangement disclosed by the Nakano patent, heat the holding furnace along a bottom wall or sidewall of the holding furnace and typically require space along the sides or bottom of the holding furnace for the heating elements. With such top/external heating arrangements, it is difficult to maintain a constant molten metal temperature in the holder furnace. In addition, with such top/external heating arrangements it is difficult to maintain a xe2x80x9ccleanxe2x80x9d supply of molten metal. These arrangements are also generally known to contribute to metal oxide formation in the molten metal.
An alternative to top/external heating arrangements is to provide bottom heating devices in holding furnaces. Such bottom heating devices are typically embedded within the bottom wall of the holding furnace. One known bottom heating arrangement in a molten metal holding furnace is disclosed by U.S. Pat. No. 5,411,240 to Rapp et al. The heating cycle of such bottom heating arrangements places significant stress on the bottom wall of the holding furnace. Such embedded arrangements are also generally unsuitable for use with containment difficult metals such as molten aluminum alloys. Any leakage of molten aluminum alloy into the bottom wall of the holding furnace will cause failure of the embedded heating elements.
In view of the foregoing, an object of the present invention is to provide a bottom heated holder furnace for containing molten metal that frees space above the holder furnace. Another object of the present invention is to provide a bottom heated holder furnace that is suitable for use in a molten metal casting system. It is another object of the present invention to provide a bottom heated holder furnace which is suitable for use with molten aluminum alloys, eliminates restriction within the holder furnace, and is less likely to cause metal quality issues.
The above objects are accomplished with a molten metal holder furnace and molten metal casting system in accordance with the present invention. The holder furnace preferably contains a supply of molten metal that may be supplied to a casting mold through a plurality of molten metal injectors. The holder furnace includes a storage vessel having sidewalls and a bottom wall defining a molten metal receiving chamber for containing the supply of molten metal. At least one furnace insulating layer lines the molten metal receiving chamber of the storage vessel. A thermally conductive heat exchanger block is located at the bottom of the molten metal receiving chamber for heating the supply of molten metal. The heat exchanger block has a top face, a bottom face, and side faces. The heat exchanger block includes a plurality of electrical heaters extending therein and projecting outward from at least one of the faces of the heat exchanger block, and further extending through the furnace insulating layer, and at least one of the sidewalls of the storage vessel for connection to a source of electrical power. A sealing layer at least partially covers the bottom face and side faces of the heat exchanger block such that the heat exchanger block is substantially separated from contact with the furnace insulating layer.
The heat exchanger block may include a plurality of individual heat exchanger blocks connected together along side faces by a tongue-in-groove connection. The storage vessel may further include a molten metal inlet for receiving the supply of molten metal into the molten metal receiving chamber from an external source, and a molten metal outlet for returning the supply of molten metal to the external source. A layer of refractory material may be located within the molten metal receiving chamber and on top of the heat exchanger block. The layer of refractory material may define a plurality of vertically extending chambers. The sealing layer may further partially cover the top face of the heat exchanger block such that the top face of the heat exchanger block is separated from contact with the layer of refractory material except on areas of the top face substantially coincident with the vertically extending chambers whereby the heat exchanger block may be in direct contact with molten metal when a supply of molten metal is contained in the storage vessel and the vertically extending chambers. The plurality of vertically extending chambers may be connected in series from the molten metal inlet to the molten metal outlet of the storage vessel.
A cover may be positioned on top of the storage vessel and substantially enclose the molten metal receiving chamber. The cover may define a plurality of openings corresponding to the plurality of vertically extending chambers for receiving, respectively, the plurality of molten metal injectors into the plurality of vertically extending chambers. A lift device may be located beneath the bottom wall of the storage vessel for lifting the holder furnace into engagement with the plurality of molten metal injectors such that the molten metal injectors extend, respectively, into the plurality of vertically extending chambers defined within the molten metal receiving chamber.
The sealing layer may further line the molten metal receiving chamber. The at least one furnace insulating layer may include a plurality of furnace insulating layers positioned between the sealing layer and the sidewalls and bottom wall of the storage vessel. The sealing layer may be an alumina fiber mat. The heat exchanger block may be made of graphite or silicon carbide.
The electrical heaters may extend between opposite sidewalls of the storage vessel and through the heat exchanger block. The electrical heaters may each include a continuous heating element extending through at least one of the opposite sidewalls, the at least one furnace insulating layer, and extending at least partially through the heat exchanger block. The electrical heaters may each further include respective tubes extending through the opposite sidewalls, the at least one furnace insulating layer, and extending at least partially into opposite faces of the heat exchanger block. The heating element for the electrical heaters may extend at least partially through each of the respective tubes. Sealing gaskets may be positioned within the heat exchanger block. The sealing gaskets may cooperate, respectively, with ends of the tubes extending into the opposite faces of the heat exchanger block for preventing molten metal from leaking into the tubes and contacting the heating element of the electrical heaters. The tubes may be ceramic insulating tubes that are substantially surrounded by a layer of ceramic fiber rope for preventing molten metal from leaking into the ceramic insulating tubes and contacting the heating element of the electrical heaters.
Flange plates may be attached, respectively, to the ceramic insulating tubes at the opposite sidewalls of the storage vessel. The ceramic insulating tubes may be held in compression against the opposite sidewalls of the storage vessel via the flange plates, bolts, and a plurality of Belleville washers stacked to yield about 175 pounds of torque on each of the ceramic insulating tubes. A source of inert gas may be in fluid communication with the heat exchanger block through the tubes such that the heating element of the electrical heaters operates substantially in an inert gas atmosphere during operation of the holder furnace.